1. Field of the Invention
This invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has positive temperature coefficient (PTC) of resistivity characteristics that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a polymer matrix is well known in the art, as disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882.
The increase in the resistance value is believed to be due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic PTC thermistor can be used as an overcurrent or overheat-protecting element, a self-regulating heater, and a temperature sensor.
Of these, the overcurrent or overheat-protecting element in series connection with an electric circuit requires a sufficiently low resistance value at room temperature in a non-operating state, a sufficiently high rate of change between the room-temperature resistance value and the resistance value in operation, and a minimal change of resistance upon repetitive operation. Conductive particles used in organic PTC thermistors are typically carbonaceous conductive particles such as carbon black and graphite. In order to reduce the resistance of the thermistor, however, a large amount of conductive particles must be blended at the expense of rate of resistance change, failing to provide satisfactory characteristics for protecting overcurrent or overheating.
This drawback can be overcome using conductive metal particles having a lower resistivity than carbonaceous particles. To find a compromise between a low room-temperature resistance and a high resistance change rate, the use of conductive metal particles having spiky protuberances has been proposed. JP-A 5-47503 discloses an organic PTC thermistor comprising, in kneaded admixture, a crystalline polymer and conductive particles having spiky protuberances. U.S. Pat. No. 5,378,407 discloses a conductive polymer composition comprising a crystalline polyolefin, olefin copolymer or fluoropolymer and Ni filaments having spiky protuberances.
Likewise, the inventors proposed in JP-A 10-214705, JP-A 11-168005, JP-A 2000-82602, and JP-A 2000-200704, the use of conductive particles having spiky protuberances to provide both a low room-temperature resistance and a high resistance change rate.
However, the inventors found that these organic PTC thermistors using conductive metal particles lack reliability in terms of long-term storage or the like. In a storage test, the room-temperature resistance gradually increased with the passage of time to an extent which was dependent on storage test conditions.
The reasons are that conductive metal particles are oxidized on their surface to reduce their conductivity, that during storage, more conductive particles agglomerate to break some conductive paths, and the like. It has been found that the problem of resistance increase can be solved by pretreating surfaces of conductive metal particles with an organic material.
An object of the invention is to improve the performance stability of an organic PTC thermistor using conductive metal particles.
The above and other objects are achieved by the present invention which is defined below.
(1) An organic positive temperature coefficient thermistor comprising, an organic polymer matrix and conductive metal particles dispersed therein, wherein the conductive metal particles have a layer of organic material disposed thereon, and wherein the organic material is different from the organic polymer matrix, does not covalently bond with the conductive metal particles, and is not compatible at a molecular level with the organic polymer matrix.
(2) The organic positive temperature coefficient thermistor of (1), wherein said organic material is not reactive with the organic polymer matrix.
(3) The organic positive temperature coefficient thermistor of (1) or (2), wherein said conductive metal particles comprise nickel or copper.
(4) The organic positive temperature coefficient thermistor of any one of (1) to (3), wherein said organic polymer matrix comprises a thermoplastic polymer.
(5) The organic positive temperature coefficient thermistor of any one of (1) to (4), further comprising a low-molecular weight organic compound.
(6) The organic positive temperature coefficient thermistor of any one of (1) to (5), wherein said conductive metal particles have spiky protuberances.
(7) The organic positive temperature coefficient thermistor of any one of (1) to (6), wherein said organic material is biodegradable.
(8) A method for preparing an organic positive temperature coefficient thermistor, comprising the steps of pretreating conductive metal particles with an organic material so that the organic material covers surfaces of the conductive metal particles, and mixing the treated conductive metal particles with an organic polymer matrix for dispersion.
(9) The method of (8), wherein said organic material is different from the organic polymer matrix, does not covalently bond with the conductive metal particles, and is not compatible at a molecular level with the organic polymer matrix.
(10) The method of (8) or (9), wherein said organic material is not reactive with the organic polymer matrix.
(11) The method of any one of (8) to (10), wherein said conductive metal particles comprise nickel or copper.
(12) The method of any one of (8) to (11), wherein said organic polymer matrix comprises a thermoplastic polymer.
(13) The method of any one of (8) to (12), wherein said organic polymer matrix further comprises a low-molecular weight organic compound.
(14) The method of any one of (8) to (13), wherein said conductive metal particles have spiky protuberances.
(15) The method of any one of (8) to (14), wherein said organic material is biodegradable.
The organic PTC thermistor of the invention is characterized in that conductive metal particles pretreated on surfaces with an organic material are dispersed in an organic polymer matrix.
The pretreatment of conductive metal particle surfaces with an organic material prevents diffusion of oxygen into particle surfaces and hence, oxidation of particles during long-term storage, thereby stabilizing the resistance value. The surface treatment is also believed effective for preventing excessive agglomeration of conductive metal particles. Additionally, the surface treatment permits the conductive metal particles to be readily separated from the organic polymer matrix.